Linear magnetostrictive actuator

ABSTRACT

Exemplary practice of the present invention provides a magnetostrictive actuator characterized by linear force output and uniform magnetic biasing. A center bias magnet drives flux through series magnetostrictive bars in opposite directions while surrounding drive coils apply flux in the same direction through the bars. The net response is substantially linear with respect to the drive coil current. A second parallel set of magnetostrictive bars completes the flux path and adds to the actuator output force. Flux leakage between the parallel bars is compensated by a ferromagnetic shunt or by a tapered magnet providing uniform flux density down the length of the magnetostrictive bars. The closed flux path allows magnetic shielding of the entire actuator, if desired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.62/564,100, filed 27 Sep. 2017, hereby incorporated herein by reference,entitled “Linear Magnetostrictive Actuator,” joint inventors John E.Miesner and George G. Zipfel.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates to magnetostrictive actuators, moreparticularly to magnetostrictive actuators that seek to produce a linearforce output and/or a uniform magnetic bias flux in the magnetostrictiveelements.

Magnetostrictive actuators offer great promise for applications thatrequire a high force output over a wide bandwidth. However,magnetostrictive materials have two characteristics that limit theiruse. The first limiting characteristic is the inherent nonlinearmaterial response in strain to magnetic flux density. Many applicationsrequire a linear force output that has not been achieved by the currentart for magnetostrictive actuators. The second limiting characteristicis the relatively low permeability of magnetostrictive materials insofaras it makes it difficult to achieve a uniform magnet bias down thelength of the magnetostrictive element due to flux leakage.

U.S. Pat. No. 5,451,821 to Teter et al., incorporated herein byreference, teaches a method of compensating for magnetic flux leakageusing magnets outside the drive coils to apply a magnetic fieldperpendicular to the desired bias direction. Teter et al.'s method hasproven to be effective and has been widely adopted in the design ofmagnetostrictive actuators. However, the perpendicular magnets requiredby Teter et al. are relatively large, thus increasing the size and costof an actuator using this method. The perpendicular magnets also causelarge magnetic fields external to the actuator, which are not acceptablein many applications. These external fields cannot be effectivelyshielded by the usual method of surrounding the entire actuator with aferromagnetic case, because doing so would short out the perpendicularmagnet flux.

U.S. Pat. No. 6,891,286 to Flanagan et al., incorporated herein byreference, teaches large axially polarized disk magnets at each end of amagnetostrictive rod to achieve a uniform magnetic flux down the lengthof the rod. However, this approach of Flanagan et al. does nothing toaddress the inherent nonlinear material response, requires largemagnets, and has large external magnetic fields.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a magnetostrictive actuator having linear force output anduniform magnetic biasing in the magnetostrictive elements.

An exemplary embodiment of the present invention is a magnetostrictiveactuator that uses parallel bars so as to produce an at leastsubstantially linear force output concomitant with compensation for fluxleakage, thereby allowing for enhanced length as compared with thecurrent state of the art. Exemplary inventive practice achieves linearforce output and uniform magnetic biasing through inventiveimplementation of a relatively small bias magnet in a closed flux loopwithout significant external fields. An exemplary parallel barmagnetostrictive actuator in accordance with the present inventionproduces a linear force output using a center bias magnet, and providescompensation for flux leakage using a ferromagnetic shunt or a taperedpermanent magnet.

U.S. Pat. No. 5,587,615 to Murray et al., incorporated herein byreference, teaches a method to linearize the output of a magneticactuator with force generated across air gaps. Murray arranges two airgaps with the total actuator force equal to the difference of the forcesacross them, and then establishes (i) magnetic bias flux in oppositedirections in the two air gaps and (ii) coil flux in the same directionin the two air gaps. Therefore, as coil flux increases it tends tocancel the bias flux in one gap and add to the bias flux in the othergap.

The inherent force generated across an air gap is quadratic with respectto the total flux across the gap. If the bias flux is Φ_(bias) and thecoil flux is Φ_(coil), then the force in one gap can be written as F=k(Φ_(bias)±Φ_(coil))², where k is a proportionality constant dependent onthe geometry. The net force in the two gaps can be written as F_(net)=k[(Φ_(bias)+Φ_(coil))²−(Φ_(bias)−Φ_(coil))²]. Simplifying this equationyields F_(net)=4k Φ_(bias)Φ_(coil). Thus, the net output force is linearwith respect to the coil flux.

By doping iron with gallium, U.S. Navy researchers created Galfenol, amaterial that has higher magnetic permeability than other giantmagnetostrictive materials and that can withstand high tensile stressthat would cause other materials to fail. Tensile stress allows longermagnetostrictive elements to be used in an actuator, as compared tocompressive stress, because tension eliminates buckling as a mode offailure.

The present inventors recognize that the higher magnetic permeability ofGalfenol enables an actuator with a parallel bar arrangement in whichmagnetic flux from surrounding coils is conducted up one Galfenol bar,across a ferromagnetic link, down a parallel Galfenol bar, and thenacross another ferromagnetic link back to the first bar in a completeloop. The parallel bar arrangement eliminates the need for a separatededicated magnetic flux return, making the resulting actuator morecompact. However, current art for magnetic biasing of parallel barGalfenol-type actuators is to use external magnets with the attendantsize, cost, and external magnetic field problems. Also, the maximumactuator length, although greatly enhanced, is still limited by fluxleakage.

The present inventors also recognize that the response of amagnetostrictive material such as Galfenol is substantially quadraticwith respect to magnetic flux through the material up to the flux levelat which it begins to saturate. The present invention uses thisquadratic response characteristic to produce a linear net output forcein a manner analogous to the method of Murray. In accordance withexemplary inventive practice, a center bias magnet drives flux throughseries magnetostrictive bars in opposite directions, while surroundingdrive coils apply flux in the same direction through the bars. A secondparallel set of magnetostrictive bars completes the flux path. The forceoutput connection is between the series bars and therefore the netoutput is the difference of the force generated in them. The outputforce is linear over the flux range for which the magnetostrictivematerial response is quadratic.

As exemplarily practiced, the present invention compensates for fluxleakage between the parallel bars by either of two inventive methods.The first inventive method for flux leakage compensation includesimplementation of ferromagnetic shunts between the magnetostrictive barsthat conduct flux parallel to the bars at the same rate as the leakageflux. Therefore, there is no net change in bar flux down the length thebar. The second inventive method for flux leakage compensation includesimplementation of tapered permanent magnets between the bars polarizedperpendicular to the bars. The magnets inject flux into the bars at thesame rate as the leakage flux. Again, there is no net change in bar fluxdown the length of the bar.

Inventive application of either flux leakage compensation method isoptional, depending on the magnetic permeability of the material usedand the distance between the parallel bars as compared to the length ofthe bars (e.g., wherein each bar has the same length). As a general rulefor Galfenol, acceptable performance may be inventively achieved withoutflux leakage compensation, as long as the bar length is less than aboutfour times the bar separation distance.

Various preferred modes of practicing the present invention include whatare referred to herein as a “first” mode of inventive practice, a“second” mode of inventive practice, a “third” mode of inventivepractice, and a “fourth” mode of inventive practice.

The first mode of inventive practice uses ferromagnetic links betweenseries magnetostrictive bars to conduct flux from the permanent magnetinto the ends of the bars, and uses the ferromagnetic shunt method offlux leakage compensation.

The second mode of inventive practice uses the same ferromagnetic linkand series bar arrangement as the first inventive mode, but uses thetapered permanent magnet method of flux leakage compensation.

The third mode of inventive practice uses continuous magnetostrictivebars, rather than series magnetostrictive bars with a ferromagnetic linktherebetween. The flux from the permanent magnet enters the side of thebar at the center. This third inventive mode uses the same ferromagneticshunt method of flux leakage compensation as the first inventive mode.

The fourth mode of inventive practice uses the same continuousmagnetostrictive bar arrangement as the third inventive mode, and thesame tapered permanent magnet method of flux leakage compensation as thesecond inventive mode.

This United States patent application is related to U.S. patentapplication Ser. No. 15/717,658, hereby incorporated herein byreference, filed 27 Sep. 2017, entitled “Magnetostrictive Actuator withCenter Bias,” sole inventor John E. Miesner.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, wherein like numbers indicatesame or similar parts or components, and wherein:

FIG. 1 is a perspective view that may be conceived to be externallyrepresentative, by way of example, of any of the first, second, third,and fourth modes of practice of the present invention.

FIG. 2 is a cross-sectional view of an embodiment exemplary of the firstmode of practice of the present invention.

FIG. 3 is an exploded view of the inventive first-mode embodiment shownin

FIG. 2.

FIG. 4 is a diagram showing an example of the calculated magnetic fluxlines with no drive current, for the inventive first-mode embodimentshown in FIG. 2.

FIG. 5 is a diagram, similar to FIG. 4, showing an example of thecalculated magnetic flux lines at maximum drive current, for theinventive first-mode embodiment shown in FIG. 2.

FIG. 6 is a graph showing an example of the calculated magnetic fluxdensity for a contour line down the center of one leg at no drivecurrent and at maximum drive current, for an inventive first-modeembodiment such as shown in FIG. 2.

FIG. 7A is a diagram showing an example of the calculated magnetic fluxlines with no drive current, for a portion of the inventive first-modeembodiment shown in FIG. 2.

FIG. 7B is a diagram, similar to FIG. 7A, showing an example of thecalculated magnetic flux lines without ferromagnetic shunts, for aportion (same portion as shown in FIG. 7A) of the inventive first-modeembodiment shown in FIG. 2.

FIG. 8 is a cross-sectional view of an embodiment exemplary of thesecond mode of practice of the present invention.

FIG. 9A is a diagram showing an example of the calculated magnetic fluxlines with no drive current, for a portion of the inventive second-modeembodiment shown in FIG. 8.

FIG. 9B is a diagram, similar to FIG. 9A, showing an example of thecalculated magnetic flux lines without permanent magnet tapers, for aportion (same portion as shown in FIG. 9A) of the inventive second-modeembodiment shown in FIG. 8.

FIG. 10 is a cross-sectional view of an embodiment exemplary of thethird mode of practice of the present invention.

FIG. 11 is an exploded view of the inventive third-mode embodiment shownin

FIG. 10.

FIG. 12 is a diagram showing an example of the calculated magnetic fluxlines with no drive current, for the inventive third-mode embodimentshown in FIG. 10.

FIG. 13 is a diagram, similar to FIG. 12, showing an example of thecalculated magnetic flux lines at maximum drive current, for theinventive third-mode embodiment shown in FIG. 10.

FIG. 14 is a graph showing an example of the calculated magnetic fluxdensity for a contour line down the center of one leg at no drivecurrent and at maximum drive current, for an inventive third-modeembodiment such as shown in FIG. 10.

FIG. 15 is a cross-sectional view of an embodiment exemplary of thefourth mode of practice the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

FIG. 1, FIG. 2, and FIG. 3 are three different views of an embodiment inaccordance with the first mode of practice of the present invention. Topleft and bottom left magnetostrictive bars 101 a and 101 c,respectively, are each connected to left center support 110 a. Top rightand bottom right magnetostrictive bars 101 b and 101 d, respectively,are each connected to right center support 110 b. Center supports 110 aand 110 b are both attached to output links 116 a and 116 b, which areattached to output shaft 103. Therefore changes in length of themagnetostrictive bars 101 a, 101 b, 101 c, and 101 d correspondinglymove output shaft 103 with respect to support frame 108, therebyproducing useful work. Center supports 110 a and blob, output links 116a and 116 b, and frame 108 are preferably made of a non-magnet highstrength material such as stainless steel. The attachment to themagnetostrictive bars may be, for instance, by bonding or welding or maybe mechanical.

Top left and bottom left magnetostrictive bars 101 a and 101 c are eachin contact with left ferromagnetic link 102 a while top right and bottomright magnetostrictive bars 101 b and 101 d are each in contact withright ferromagnetic link 102 b. Top left and top right magnetostrictivebars 101 a and 101 b are each in contact with top ferromagnetic link 114a while bottom left and bottom right magnetostrictive bars 101 c and 101d are each in contact with bottom ferromagnetic link 114 b. Therefore, aclosed magnetic flux conducting loop is formed by magnetostrictive bars101 a, 101 b, 101 c, and 101 d and ferromagnetic links 102 a, 102 b, 114a, and 114 b.

Each magnetostrictive bar is surrounded by a drive coil. Drive coil 105a surrounds magnetostrictive bar 101 a; drive coil 105 b surroundsmagnetostrictive bar 101 b; drive coil 105 c surrounds magnetostrictivebar 101 c; and drive coil 105 d surrounds magnetostrictive bar 101 d.The drive coils are all wired in a combination of series or parallel asdesired such that each coil carries the same amount of current and theflux adds around the flux conducting loop. Thus, left magnetostrictivebars 101 a and 101 c always have drive coil flux in same direction, andright magnetostrictive bars 101 b and 101 d also always have drive coilflux in the same direction.

Bias magnet 111 is polarized in the transverse direction and is incontact with left and right ferromagnetic shunts 112 a and 112 b, whichare in contact with left and right ferromagnetic links 102 a and 102 b,respectively. Magnetic flux will flow from one end of bias magnet 111back to the other end with essentially equal flux following an upwardloop which includes top magnetostrictive bars 101 a and 101 b, and adownward loop which includes bottom magnetostrictive bars 101 c and 101d. Thus, left magnetostrictive bars 101 a and 101 c have bias flux inopposite directions from each other, and right magnetostrictive bars 101b and 101 d also have bias flux in opposite directions from each other.

Optimum actuator output is obtained whenever the magnetostrictive bars101 a, 101 b, 101 c, and 101 d are under preload tension. In the firstmode of practice of the present invention, this tension is provided bypreload springs 106 a and 106 b, which press upward on preload bolts 104a and 104 b, respectively. Preload bolts 104 a and 104 b are connectedto top support 114, which is connected to top magnetostrictive bars 101a and 101 b. Bottom magnetostrictive bars 101 c and 101 c 1 areconnected to bottom support 113, which is connected to frame 108. Thus,the total preload is transferred from the top to the bottom of frame108. Top and bottom supports 114 and 113 are preferably made ofnon-magnet high strength material such as stainless steel. Theattachment to the magnetostrictive bars may be by bonding or welding ormay be mechanical. Note that in inventive practice many other methods ofapplying a tensile stress are possible, depending on themagnetostrictive bar length and cross-section. For example, withrelatively long and thin bars the preload springs 106 a and 106 b areusually not required, and load can be applied directly between the frame108 and top support 114. In this case, the magnetostrictive barsthemselves serve as the compliant elements.

FIG. 4 shows an example of a two-dimensional magnetic model of the firstmode of practice of the present invention under condition of no currentflow through the drive coils. The magnetostrictive material is Galfenolfor the example shown in FIG. 4. The calculated magnetic flux paths areillustrated by lines F. With no drive coil current flow, all flux linesF flow from bias magnet 111 and form closed upper and lower loops backto bias magnet 111. It can be seen in FIG. 4 that left magnetostrictivebars 101 a and 101 c have bias flux in opposite directions from eachother, and right magnetostrictive bars 101 b and 101 c 1 also have biasflux in opposite directions from each other.

FIG. 5 shows the same two-dimensional magnetic model as FIG. 4 but undercondition of maximum rated current flow through the drive coils. In theexample shown in FIG. 5, the magnetic fluxes from the drive coils andfrom bias magnet 111 reinforce in the top magnetostrictive bars 101 aand 101 b and cancel in the bottom magnetostrictive bars 101 c and 101d. Thus, the top magnetostrictive bars 101 a and 101 b elongate and thebottom magnetostrictive bars 101 c and 101 d shorten with respect to theno drive current condition moving center supports 102 a and 102 b in thedownward direction. If the direction of current flow is reversed, thenthe magnetic flux from the drive coils and from bias magnet 111 willcancel in the top magnetostrictive bars 101 a and 101 b and reinforce inthe bottom magnetostrictive bars 101 c and 101 d, moving center supports102 a and 102 b in the upward direction.

FIG. 6 is a plot of the calculated flux density from the FIG. 4 and FIG.5 examples for a vertical contour through the center of the leftmagnetostrictive bars 101 a and 101 c. It can here be seen that undercondition of no current flow through the drive coils the bias flux isnearly constant at 0.6 Tesla down the length of the bars. Undercondition of maximum rated current flow, the magnetic flux density inthe left bottom bar 101 a is nearly zero while the magnetic flux densityin the left top bar 101 c is about 1.2 Tesla. If the direction ofcurrent flow were reversed these flux density values would also reverse.

FIG. 7A and FIG. 7B may be compared. FIG. 7A illustrates the calculatedmagnetic flux lines with no drive current for a portion of the firstmode of the present invention. In contrast, FIG. 7B illustrates thecalculated magnetic flux lines under the same conditions withoutferromagnetic shunts 112 a and 112 b. Note that without theferromagnetic shunts 112 a and 112 b, flux leakage between topmagnetostrictive bars 101 a and 101 b causes the number of flux lines toreduce with distance from bias magnet 111, which indicates that themagnetic flux density is decreasing. With ferromagnetic shunts 112 a and112 b in place, the number of flux lines is constant in topmagnetostrictive bars 101 a and 101 b, indicating a uniform magneticflux density.

In the light of the instant disclosure, the shape of ferromagneticshunts 112 a and 112 b may be calculated by a person having ordinaryskill in the art using a magnetic model and adjusting geometricparameters until the flux is at the desired level and within acceptablebounds of uniformity. The optimum shape of ferromagnetic shunts 112 aand 112 b depends upon the magnetic permeability of the magnetostrictivematerial and is a compromise because the permeability varies withmagnetic flux level and stress. As a general guideline, a wedge with aconstant taper angle and constant gap from magnetostrictive bars 101 aand 101 b, such as shown in FIG. 7B, provides good results for a widerange of magnetic permeability.

FIG. 8 is a cross-section view of an example of the second mode ofpractice of the present invention. Everything remains the same as in thefirst mode of practice of the present invention, except that theferromagnetic shunts 112 a and 112 b have been replaced with permanentmagnet tapers 212 a, 212 b, 212 c and 212 d and bias magnet 111 has beenreplaced with bias magnet 211, which performs the same function but maybe a different size. Permanent magnet tapers 212 a, 212 b, 212 c and 212d, shown in FIG. 8, are polarized in the same direction as bias magnet211.

FIG. 9A and FIG. 9B may be compared. FIG. 9A shows the calculatedmagnetic flux lines with no drive current for a portion of the secondembodiment of the present invention. In contrast, FIG. 9B shows thecalculated magnetic flux lines under the same conditions withoutpermanent magnet tapers. Note that without the permanent magnet tapers,flux leakage between top magnetostrictive bars 101 a and 101 b causesthe number of flux lines to reduce with distance from bias magnet 111,which indicates that the magnetic flux density is decreasing. Withpermanent magnet tapers 212 a, 212 b, 212 c and 212 d in place, thenumber of flux lines is constant in top magnetostrictive bars 101 a and101 b, indicating a uniform magnetic flux density.

In the light of the instant disclosure, the shape of permanent magnettapers 212 a, 212 b, 212 c and 212 d may be calculated by a personhaving ordinary skill in the art using a magnetic model and adjustinggeometric parameters until the flux is at the desired level and withinacceptable bounds of uniformity. The optimum shape of permanent magnettapers depends upon the magnetic permeability of the magnetostrictivematerial and is a compromise because the permeability varies withmagnetic flux level and stress. As a general guideline, a wedge with aconstant taper angle and constant gap from magnetostrictive bars 101 aand 101 b, such as shown on FIG. 9A, provides good results for a widerange of magnetic permeability.

FIG. 10 and FIG. 11 are two views of an example of the third mode ofpractice of the present invention. The left and right series arrangementof magnetostrictive bars in the first and second embodiments has beenreplaced with single left and right continuous magnetostrictive bars 301a and 301 b. Center supports 110 a and 110 b have been replaced withcenter attachments 310 a, 310 b, 310 c, and 310 d that attach directlyto the sides of magnetostrictive bars 301 a and 301 b and to outputlinks 116 a and 116 b. The attachment to the magnetostrictive bars maybe by bonding or welding or may be mechanical.

FIG. 12 shows an example of a two-dimensional magnetic model of thethird mode of practice of the present invention under condition of nocurrent flow through the drive coils. The magnetostrictive material isGalfenol for the example shown in FIG. 12. The calculated magnetic fluxpaths are illustrated by lines F. With no drive coil current flow, allflux lines F flow from bias magnet 311 and form closed upper and lowerloops back to bias magnet 311. It can be seen in FIG. 12 that the tophalf and the bottom half of magnetostrictive element 301 a have biasflux in opposite directions from each other, and also that the top halfand the bottom half of magnetostrictive element 301 b have bias flux inopposite directions from each other.

FIG. 13 shows the same two-dimensional magnetic model as FIG. 12, butunder condition of maximum rated current flow through the drive coils.In the example shown in FIG. 13, the magnetic fluxes from the drivecoils and from bias magnet 311 reinforce in the top halves ofmagnetostrictive bars 301 a and 301 b and cancel in the bottom halves.Thus, the top halves of magnetostrictive bars 301 a and 301 b elongateand the bottom halves shorten with respect to the no drive currentcondition, moving attachments 310 a, 310 b, 310 c, and 310 d in thedownward direction. If the direction of current flow is reversed thenthe magnetic flux from the drive coils and from bias magnet 311 willcancel in the top halves of magnetostrictive bars 301 a and 301 b andreinforce in the bottom halves, moving center attachments 310 a, 310 b,310 c, and 310 d in the upward direction.

FIG. 14 is a plot of the calculated flux density from the FIG. 12 andFIG. 13 examples for a vertical contour through the center of the leftmagnetostrictive bar 301 a. It can be seen in FIG. 14 that undercondition of no current flow through the drive coils the bias flux isnearly constant at 0.6 Tesla down the length of the bar except near thecenter. Under condition of maximum rated current flow the magnetic fluxdensity in the bottom half of left magnetostrictive bar 301 a is nearlyzero while the magnetic flux density in the top half is about 1.2 Tesla.If the direction of current flow were reversed these flux density valueswould also reverse.

FIG. 15 is a cross-section view of an example of the fourth mode ofpractice of the present invention. Everything remains the same as in thethird mode of practice of the present invention, except that theferromagnetic shunts 312 a and 312 b have been replaced with permanentmagnet tapers 412 a, 412 b, 412 c and 412 b, and bias magnet 311 hasbeen replaced with bias magnet 411, which performs the same function butmay be a different size. Permanent magnet tapers 412 a and 412 b arepolarized in the same direction as bias magnet 411.

The present invention, which is disclosed herein, is not to be limitedby the embodiments described or illustrated herein, which are given byway of example and not of limitation. Other embodiments of the presentinvention will be apparent to those skilled in the art from aconsideration of the instant disclosure, or from practice of the presentinvention. Various omissions, modifications, and changes to theprinciples disclosed herein may be made by one skilled in the artwithout departing from the true scope and spirit of the presentinvention, which is indicated by the following claims.

What is claimed is:
 1. A magnetostriction-based actuator comprising: twoparallel linear magnetostrictive units; at least two ferromagnetic endmembers joining the linear magnetostrictive units at the upper and lowerends respectively of the linear magnetostrictive units; a magnetic unitsituate between the parallel linear magnetostrictive units andintermediate the upper and lower respective ends of the linearmagnetostrictive units; and two pairs of separate coaxial drive coils,each pair of drive coils partially encircling a different one of thelinear magnetostrictive units; wherein upper and lower magnetic fluxcircuits are associated with connection of the magnetic unit to upperand lower portions respectively of the linear magnetostrictive units;and wherein magnetic flux manifestations associated with electrificationof the two pairs of drive coils combine with the upper and lowermagnetic flux circuits so as to augment one of the magnetic fluxcircuits and at least substantially neutralize the other of the magneticflux circuits.
 2. The magnetostriction-based actuator of claim 1,wherein augmentation of a magnetic flux circuit is associated withincrease in length of the corresponding portions of the linearmagnetostrictive units, and wherein at least substantial neutralizationof a magnetic flux circuit is associated with decrease in length of thecorresponding portions of the linear magnetostrictive units.
 3. Themagnetostriction-based actuator of claim 1, wherein each of the linearmagnetostrictive units includes a magnetostrictive bar having an upperend and a lower end, and wherein the magnetic unit includes a magnetconnected to each magnetostrictive bar.
 4. The magnetostriction-basedactuator of claim 1, wherein: each of the linear magnetostrictive unitsincludes an upper magnetostrictive bar and a lower coaxialmagnetostrictive bar; the magnetic unit includes a magnet and twoferromagnetic intermediate members connected to the magnet on oppositesides of the magnet; one of the ferromagnetic intermediate members isconnected between the upper and lower magnetostrictive bars of one ofthe linear magnetostrictive units; the other of the ferromagneticintermediate members is connected between the upper and lowermagnetostrictive bars of the other of the linear magnetostrictive units.5. The magnetostriction-based actuator of claim 1, further comprisingtwo flux leakage compensation units, one of the flux leakagecompensation units coupled with one of the linear magnetostrictiveunits, the other of the flux leakage compensation units coupled with theother of the linear magnetostrictive units.
 6. Themagnetostriction-based actuator of claim 5, wherein each of the fluxleakage compensation units includes at least one ferromagnetic shunt. 7.The magnetostriction-based actuator of claim 5, wherein each of the fluxleakage compensation units includes at least one magnetic taper.
 8. Amagnetostrictive actuator comprising: two parallel magnetostrictivestructures; a central permanent magnetic structure interposed betweenand contacting said two parallel magnetostrictive structures; twoferromagnetic structures respectively connecting said two parallelmagnetostrictive structures at respective upper ends and at respectivelower ends of said two parallel magnetostrictive structures; a firstpair of coaxial drive coils separated from each other and surroundingupper and lower portions respectively of a first said magnetostrictivestructure; a second pair of coaxial drive coils separated from eachother and surrounding upper and lower portions respectively of a secondsaid magnetostrictive structure; wherein an upper closed magnetic fluxconducting loop is formed by said central permanent magnetic structure,respective said upper portions of said two parallel magnetostrictivestructures, and the upper said ferromagnetic structure; wherein a lowerclosed magnetic flux conducting loop is formed by said central permanentmagnetic structure, respective said lower portions of said two parallelmagnetostrictive structures, and the lower said ferromagnetic structure;wherein magnetic fluxes resulting from application of drive current tosaid first and second pairs of said drive coils reinforce one of saidupper closed magnetic flux conducting loop and said lower closedmagnetic flux conducting loop, and at least substantially cancel theother of said upper closed magnetic flux conducting loop and said lowerclosed magnetic flux conducting loop.
 9. The magnetostrictive actuatorof claim 8, wherein the respective said portions of said two parallelmagnetostrictive structures that are in the reinforced said closedmagnetic flux conducting loop lengthen, and the respective said portionsof said two parallel magnetostrictive structures that are in the atleast substantially canceled said closed magnetic flux conducting loopshorten.
 10. The magnetostrictive actuator of claim 9, wherein each ofsaid two parallel magnetostrictive structures is made of Galfenol. 11.The magnetostrictive actuator of claim 9, wherein each of said twoparallel magnetostrictive structures is a linear elongate structure. 12.The magnetostrictive actuator of claim 9, wherein said magneticstructure is medially interposed between said two parallelmagnetostrictive structures.
 13. The magnetostrictive actuator of claim9, further comprising at least one magnetic taper for providingcompensation for flux leakage, each said magnetic taper associated withone of said two parallel magnetostrictive structures.
 14. Themagnetostrictive actuator of claim 9, further comprising at least oneferromagnetic shunt for providing compensation for flux leakage, eachsaid ferromagnetic shunt associated with one of said two parallelmagnetostrictive structures.